The standard response for a deliver_sm that is accepted by your platform contains command status: 0x00000000 . This means that the notification has been successfully passed onto your system.
If your platform is unable to accept the deliver_sm from Nexmo, these are the statuses from the SMPP standard table that will be considered as temporary and will trigger a retry from us:
Any other response code that you submit, would be considered as Final and would not trigger any retries from Nexmo.
Details on these command statuses can be found on page 112 of the SMPP 3.4. spec document.